The statements in this section may serve as a background to help understand the invention and its application and uses, but may not constitute prior art.
Compared with conventional power devices made of silicon, Group III-Nitride (III-N) semiconductors possess a number of excellent electronic properties that enable the fabrication of modern power electronic devices and structures for use in a variety of applications. Silicon's limited critical electric field and relatively high resistance make currently available commercial power devices, circuits, and systems bulky, heavy, with further constraints on operating frequencies. On the other hand, higher critical electric field and higher electron density and mobility of III-N materials allow high-current, high-voltage, high-power and/or high-frequency performances of improved power transistors that are greatly desirable for advanced transportation systems, high-efficiency electricity generation and conversion systems, and energy delivery networks. Such systems rely on efficient converters to step-up or step-down electric voltages, and use power transistors capable of blocking large voltages and/or carrying large currents. For example, power transistors with blocking voltages of more than 500V are used in hybrid vehicles to convert DC power from the batteries to AC power. Some other exemplary applications of power transistors include power supplies, automotive electronics, automated factory equipment, motor controls, traction motor drives, high voltage direct current (HVDC) electronics, lamp ballasts, telecommunication circuits and display drives.
In spite of the enormous potential of III-N semiconductor devices for producing high-efficiency power electronics such as power amplifiers and converters, silicon-based control circuits are still necessary for integrated circuit design for power electronic devices. To enhance the utility of III-N devices, there is a critical need for monolithic integration of III-N transistors with different threshold voltages, especially enhancement-mode (E-mode) and depletion mode (D-mode) transistors. For example, an integrated E/D mode GaN logic circuit may replace a separate, conventional, silicon logic chip. Such monolithic integration of III-N transistors with different threshold voltages may allow the addition of digital or control functions to analog and mix-signal components on a common substrate, thus improving the performance of the resulting integrated circuits, while also providing design flexibility to reduce production cost and circuit foot-print. Accurate and flexible control of threshold voltages for different III-N transistors on a common substrate is also highly desirable. To achieve these implementation and integration objectives, careful technological developments are needed to determine optimal semiconductor material compositions, device structures, and fabrication processes.
For example, an important technology for use in fabricating normally-off E-mode field effect transistors for power switching applications is gate recess. Chlorine-based dry plasma etching is typically used to form gate recesses in AlGaN/GaN devices, as both GaN and AlGaN are very inert to wet chemical etchants. However, dry plasma etching is prone to plasma-induced damage and etch-based process variations. Plasma damage creates a high density of defect states and degrades channel mobility in the recessed region. Variations in the plasma etch rate make it difficult to control recess depth precisely by timed etching, which causes a variation in transistor parameters such as the transconductance and threshold voltage. Etching rates can further vary for different transistor gate lengths and/or aspect ratios. Thus, dry plasma etching-based gate recess techniques are insufficient for the integration of different types of transistors with different target threshold voltages on the same substrate.
Therefore, in view of the aforementioned practicalities and difficulties, there is an unsolved need to monolithically integrate III-N transistors with different threshold voltages on a common substrate. It is against this background that various embodiments of the present invention were developed.